This invention relates to an electrostatic chuck and more particularly to an electrostatic chuck capable of securely holding, in a flat configuration, such object to be chucked or attracted as a semiconductor wafer (hereinafter called a wafer) which inherently flexes upwardly or downwardly.
As shown in FIG. 1, a prior art electrostatic chuck comprises a flat plate shaped electrode 1, and a thin insulating dielectric film 2 applied on the surface of the electrode 1. A thin semiconductor wafer 3 is mounted on the surface of the dielectric film 2 and a high DC voltage is applied across the wafer 3 and the electrode 1 from a source 4 to electrostatically attract the wafer 3 against the surface of the dielectric film 2.
Recently, a silicon rod is expitaxially grown by vapor phase growing technique and thin silicon wafers are prepared by slicing the silicon rod and then polishing. Since the wafer is thin and has a diameter of 10 cm or more, the wafer is not perfectly flat and usually flexes more or less upwardly or downwardly. In other words, the wafer is concave or convex so that it is difficult to attract the wafer against the surface of the dielectric film in perfectly flat state. The attractive force of the electrostatic chuck is expressed by the following equation: ##EQU1## where K is a constant, n the dielectric constant of the dielectric film 2, V the voltage impressed across the electrode 1 and the wafer 3, a the thickness of the dielectric film 2, and b the gap between the dielectric film 2 and wafer 3.
As can be noted from this equation the attractive force f increases as the gap b decreases. Accordingly, when the wafer 3 is concave as shown in FIG. 2, there is no problem, but when the wafer is convex as shown in FIG. 3 or its central portion projects upwardly than the peripheral portion as shown in FIG. 3, as the peripheral portion is close or in direct contact with the surface of the dielectric film 2, the peripheral portion would firstly be attracted and secured, and the central portion of the wafer would then be attracted. For this reason, the peripheral portion must slide outwardly. However, since the peripheral portion has already been firmly attracted, it cannot slide outwardly, thus failing to bring the central portion of the wafer into firm contact in a flat state. In other words, it is impossible to correct the flexure of the wafer 3. The result of the inventor's experiment showed that, when the wafer is convex as shown in FIG. 1, the flexing of the wafer can be removed by causing electrode 1' to oppose only the central portion of the wafer 3 as shown in FIG. 3. However, such electrostatic chuck cannot attract in a flat state a concave wafer 3 as shown in FIG. 4. In FIGS. 3 and 4, reference character 4A designates an insulating substrate.